Cooking processes such as grilling, frying, baking, broiling and other similar processes generate and release into the air substantial amounts of heat and cooking by-products including grease particles, smoke, odors, and the like. In order to comply with local municipal codes as well as to assure health, safety, and cleanliness, the heated air is conventionally removed from a commercial, institutional, or industrial kitchen facility and the building housing such kitchen facility through an exhaust ventilation system.
The exhaust ventilation system typically includes a hood extending over the area in which food is cooked and a motor-driven exhaust fan for drawing air containing the aforementioned heat and cooking by-products to the hood. In turn, the drawn air is impelled by the fan to the exterior of the building.
A conventional exhaust ventilation system draws a considerable quantity of air from the interior of the building while removing the unwanted heat and cooking by-products. As should be understood, such air has likely been heated or cooled by an air conditioning system associated with the building. The removal of such air causes exterior air to be drawn into the building to replace the exhausted air, and the air conditioning system must be operated to heat or cool the drawn exterior air. The operation of the air conditioning system to continually heat or cool air which will be drawn out of the building by the exhaust ventilation system creates a substantial expense of operation through higher utility bills. Accordingly, it is useful to operate an exhaust ventilation system at a slower rate during periods of slower activity to reduce such utility bills.
Typically, a commercial, institutional, or industrial exhaust ventilation system employs one or more single-phase AC induction motors to drive one or more fans either directly or indirectly via a belt and pulley system or the like. As is well known, there are many methods of controlling the speed of an AC induction motor. Frequency inverters are common for three-phase motors but are too costly for small single-phase equipment. Usually, the rotational speed of a single-phase motor is regulated using a controller that turns off the AC voltage applied to the winding of the motor for an adjustable period during each half cycle of alternating current. However, it has been recognized that single-phase motors, including shaded pole and permanent split capacitor motors, typically have limited starting torque. Accordingly, such motors are not able to start themselves when they are connected to high starting loads and the controller is set to apply only a relatively small portion of the AC voltage to the winding of the motor. Moreover, the powered winding of a powered motor that is not rotating can burn out.
Single-phase motors that are designed to overcome the starting torque problem, such as capacitor start motors, have an extra winding that is controlled by a centrifugal switch. The switch operates to apply power to the extra winding during start-up, and then cuts out the extra winding when the motor achieves sufficient running speed. However, a reduction in speed in such a motor causes the switch to close and activate the extra winding, resulting in excessive temperature, speed fluctuation, and possible winding burnout.
Accordingly, it is seen that a need exists for an apparatus and method for starting such a motor, for controlling the motor during a period of normal operation, and for allowing the motor to rotate at a relatively low speed during the period of normal operation while avoiding the problems outlined above.